Synchronizing printheads in order to correct for printing inaccuracies is a necessity in most printing systems since mechanical systems invariably include some sort of deviation from their desired target. For example, U.S. Pat. No. 6,068,362 ('362 patent) discloses a method for synchronizing printheads of a printing system. The printing system includes a plurality of printheads with optical sensors mounted “before” each printhead (upstream) at some predetermined distance. (see column 9, line 60 through column 10, line 4 of the '362 patent) A print media or a conveyor belt passes beneath the printheads in order to permit the printheads to print marks thereon. The optical sensors capture an image of the marks which are input into a synchronization circuit. The synchronization circuit determines whether any deviation from the desired target is present. If there is a deviation, the synchronization circuit modifies the line spacing of the printhead of interest in order to compensate for the inaccuracies. In this system, the adjusted line spacings are based on an output of an encoder attached to the paper drive motor. Such a system requires extremely high cost encoders to provide the resolution needed for the registration demands of a printer system. It also is subject to errors associated with slip or coupling between the motor and the motion of the paper through the print zone. This system is also very susceptible to errors produced by variations in motor speed such as wow and flutter.
It is noted that the above-described system discloses the printheads disposed spatially ahead of the particular printhead to which it is associated. In this configuration, there is an inherent time lag from image capture until the media passes beneath the printhead. This time lag in and of itself introduces another variable which is also subject to deviation from its desired target.
European Patent Application EP 0 729 846 A2 discloses a printed reference image compensation system. Similar to the '362 patent, there are a plurality of printheads for printing cue marks as the print media passes beneath each printhead. A camera “before” the second printhead captures an image of the cue mark printed by the first printhead. This permits the second printhead to adjust its printing if a deviation is detected as discerned from the captured image. More specifically, it states in column 7, lines 4-7, “the cue mark 18 must be sensed sufficiently in advance of the subsequent printhead 46 to allow the control signal from sensor 22 to be used to initiate the start of print by head 26 at the proper instant in time.” Similar to the '362 patent, there is an inherent time lag between image capture and subsequent printing by the particular printhead which is undesirable as stated hereinabove.
Consequently, a need exists for a printing system which overcomes the above-described drawbacks.